# What input voltage should I see

Registered Users Posts: 20✭✭
I have an Rv solar setup. 4 x 85 watt 36 cell panels wired series. Specs are Imp 4.93a /Vmp 17.3v
I have the panels wired
to a Midnite combiner box and then to a Midnite kid controller. Battery bank is 4 T 105's
What imput voltage should I see on the controller if I have this wired correctly??
2 strings of 2 85 watt panels on 10 amp DC fuses (2)
Mike

• Banned Posts: 17,615✭✭✭
Re: What input voltage should I see

If all panels are wired in series you will see up to Vmp * the number of panels or 69.2 Volts with a load on the controller. MPPT controllers will pick their own 'best' Vmp * Imp and panels are a current source and will let Voltage go to any level providing they can put out their Imp.

I don't know what you mean by "two strings on 10 Amp fuses". Two strings of panels? Two in each string? Or two strings of four each? Or two strings of T105's in parallel?

BTW if it is four 85 Watt panels that will be 340 Watts and put out about 22 Amps on a 12 Volt system. That is half what is needed for two parallel strings of T105's @ 225 Amp hours each. As such that array is not sufficient for the battery bank (450 Amp hours total; 22 Amps is <5%).
• Registered Users Posts: 20✭✭
Re: What input voltage should I see

I have 2 more of the same panels ... So then I'd have 3 strings of 2 panels totalling 510 watts
is that going to be enough for the battery bank?
Thanx
Mike
• Banned Posts: 17,615✭✭✭
Re: What input voltage should I see

510 Watts should be capable of 32 Amps @ 12 Volts. But the Kid will limit it to 30 Amps, which is just 6.6% on 450 Amp hours.

Is it necessary to stay at 12 Volts for the system? If you go up to 24 you will improve things for the batteries (only one string), increase the amount of power the Kid can transfer (30 Amps @ 24 Volts - up to 900ish Watts), and with enough panel improve the charge rate. The existing 85 Watt panel strings can be paralleled with another string of different value so long as the Vmp's are close.
• Registered Users Posts: 20✭✭
Re: What input voltage should I see
510 Watts should be capable of 32 Amps @ 12 Volts. But the Kid will limit it to 30 Amps, which is just 6.6% on 450 Amp hours.

Is it necessary to stay at 12 Volts for the system? If you go up to 24 you will improve things for the batteries (only one string), increase the amount of power the Kid can transfer (30 Amps @ 24 Volts - up to 900ish Watts), and with enough panel improve the charge rate. The existing 85 Watt panel strings can be paralleled with another string of different value so long as the Vmp's are close.
The trailer is full of 12 volt LED lights. I also have a 3 KW pure sine inverter....12volt.
Using the combiner box can I bring down a larger string of panels say 2 x 250 or 1 x 250 watt or is it too much difference between the (36 cell 85 watt ) and a 60 cell or 72 cell panel?
I know Ive been told about mixing panels of various voltages...the larger panels will just be reduced to the smallest panel wattage wise.
• Banned Posts: 17,615✭✭✭
Re: What input voltage should I see
The trailer is full of 12 volt LED lights. I also have a 3 KW pure sine inverter....12volt.
Using the combiner box can I bring down a larger string of panels say 2 x 250 or 1 x 250 watt or is it too much difference between the (36 cell 85 watt ) and a 60 cell or 72 cell panel?
I know Ive been told about mixing panels of various voltages...the larger panels will just be reduced to the smallest panel wattage wise.

The problem you have is that the 30 Amp limit of the Kid on a 12 Volt system means the best charge rate you can achieve on 450 Amp hour batteries is (30 * 100 / 450) 6.6%. This is very minimal and would work if no or very minimal loads are on while the batteries are charging, but that isn't likely. The 510 Watts of panel you have now would max out the controller; adding more panel will not work on the same controller.

The Voltage difference between two 17.5 Vmp panels in series and one 35 Vmp (72 cell) is nil, but the controller issue makes it irrelevant. (The 60 cell panel would have a Vmp around 30 and would not work well with an array Vmp of 35 BTW.)

So if you want to add another 15 Amps of charging power on the 12 Volt system you need a second controller as well as more panel. It doesn't have to be an MPPT controller if the panels are around the right Voltage. For example two 130 Watt panels in parallel on a 20 Amp PWM type controller.
• Registered Users Posts: 20✭✭
Re: What input voltage should I see

i just upgraded my CC TO THE KID...So i do have the other 30 amp pwm controller just lying about... perhaps i could have two arrays on two controllers.
It just keeps on getting better!
• Banned Posts: 17,615✭✭✭
Re: What input voltage should I see
i just upgraded my CC TO THE KID...So i do have the other 30 amp pwm controller just lying about... perhaps i could have two arrays on two controllers.
It just keeps on getting better!

Good. Especially if you get the extra Watts from less expensive GT style panels, put them on the Kid, and put the standard 85 Watt panels on the old controller. You get the bigger array & the charging needed for the batteries without having to spend a lot for more PV (GT style panels tend to be 200 Watts or more and half the price per Watt of standard panels, but you need the MPPT function to make use of the Voltage difference).